Bisphenol-A (BPA) is a cross-linker for synthetic resins used as coatings, and began to replace resins based on natural oils (oleoresins) in the mid-1970s. BPA-based coatings have high corrosion resistance compared to oleoresins and are widely used, e.g., in food packaging. In the United States, over 300 billion beer, beverage, and food cans are coated with half a million metric tons of BPA-containing epoxy resins each year, and the global market is more than twice that large. Although there are currently no U.S. Food and Drug Administration (FDA) or other U.S. regulatory restrictions on the use of BPA-based resins in most food containers, BPA-related health hazards have been recognized by regulators, policymakers, and consumers. Controversy over health implications has caused concern over the use of BPA in food packaging. BPA is banned from use in applications such as infant feeding plastic bottles, and California recently listed BPA as a hazardous material.
There is much interest in cost-effective and functional replacements for BPA-based epoxy resins in can coatings that may contact food. Desirable characteristics for alternative coatings are numerous and challenging, including coating integrity (adhesion, strength, flexibility, pH/corrosion resistance, and the like) under sterilization, handling, and storage, no effect on food taste, compliance with FDA guidelines on direct food contact use, cost-effective, compatible with established manufacturing processes, and the like.
Many attempts to develop a viable solution have been made. Natural oils may be functionalized with hydroxyl or carboxyl groups and may be converted to polyesters and polyurethanes for use in coatings, inks, adhesives, foams, and the like. However, oleoresins often exhibit poor corrosion resistance. For example, it is believed that acidic tomato juice readily damages oleoresin coatings. Chemistries such as vinylation, acrylation, polyesterifcation, polyolefinination, and use of a variety of cross-linkers have been explored, but have not been successful because of failure in one or more desirable characteristics, such as flexibility, adhesion, application method, cure speed, corrosion resistance, or hydrolysis under low pH.
Some epoxy-based resin alternatives have been investigated using alternative cross-linkers, such as diglycidyl ethers of n-alkyl diphenolates, isosorbide, and bisguaiacol. However, these alternatives are costly and have been reported to suffer from problems such as estrogen receptor activity, epichlorohydrin toxicity, and poor hydrolytic stability.
The present application appreciates that developing corrosion resistant resins. e.g., for replacing BPA-cross-linked resins in can coatings, may be a challenging endeavor.